The chimney intersects the roof slope at a specific angle. That is why the flashing on each side sits in a different position. The lower apron directs water onto the covering, the side sections guide it along the walls, and the upper section must handle the runoff from the ridge side. The measurement covers more than the chimney perimeter. The heights at the roof slope, its pitch and the connection plan are also needed. A sketch prepared in this way reduces the number of corrections that have to be made on the roof. Dimensions should be referenced to the final covering surface. A measurement taken before the battens or the sheet profile are laid may give a different height for the detail. On the sketch it is worth marking the slope line and the direction to the ridge. This allows the front and rear sides of the chimney to be clearly identified.

Flashing around a chimney on a metal roof must take over the water flowing down the slope and direct it safely onto the covering, while allowing for roof movement and the chimney construction. Simply applying tape or sealant is not enough. Check the geometry of the detail, the order of laps, the fit to the sheet profile and the connection to the chimney. Pay attention to drainage behind the chimney, fixings in the water-flow zone and the termination of the flashing at the vertical part of the chimney.

A valley gutter is the concave junction of two roof slopes that collects water from both sides and directs it to the eaves. When designing it, the key factor is the width of the exposed channel, matched to the volume of flowing water and the length of the drainage path. Check that the flashing has stable support in line with the roof system and does not deflect between fixing points. Pay attention to membrane layout with laps following the slope and to fixings kept outside the water flow path. Leave final design decisions to the system installer.